In the Battle of Buxar, Mir Qasim was supported by Shah Alam II, who is the correct answer (option 4). Shah Alam II was the Mughal emperor and he aligned himself with Mir Qasim in this battle.

To understand the other options:

1) Haider Ali (option 1) was the ruler of the Kingdom of Mysore and he did not support Mir Qasim in the Battle of Buxar.

2) Maharaja Ranjit Singh (option 2) was the ruler of the Sikh Empire and he was not involved in the Battle of Buxar.

3) Malhar Rao Holkar (option 3) was a Maratha warrior and he also did not support Mir Qasim in the Battle of Buxar.

It is important to note that in historical battles, alliances sometimes changed based on political situations and interests, therefore, it is crucial to study the specific context and circumstances of each battle to understand the alliances involved.